Between
the mid-1600s and the early 1700s, European winter temperatures
were reduced by 1.8 to 2.7 degrees Fahrenheit  and that
difference was enough to routinely freeze canals solid in
Holland, as well as lock up Greenland in ice. Rivers in Europe
that were typically ice-free, froze over and inspired Frost
Fairs as well as activities like skating and even golf
as seen here inSports on a Frozen River,ca. 1660,
by Aert van der Neer (Dutch, 1603/41677).

Photo
by Tom Cogill

Environmental Sciences professor Michael Mann has used
a variety of creative strategies to create a record that can
function as a baseline for creating climate change models
able to distinguish between ordinary oscillations in temperature
and those produced by global warming. He has tapped the information
contained in such natural archives as tree rings, ice cores
and coral reefs.

By Charles Feigenoff

Predicting
the weather a day at a time is hard enough. The problem that Michael
Mann, assistant professor of environmental
sciences, has created for himself is even more difficult:
to project the weather back year by year for a millennium and
beyond. His interest is not simply historical, though Mann is
clearly motivated in part by his curiosity about the past. Instead,
his purpose is to create a record that is lengthy enough to function
as a baseline for creating climate change models that can distinguish
between ordinary oscillations in temperature and those produced
by global warming.

Mann
has had to use a variety of creative strategies to produce such
a record. First of all, he has tapped the information contained
in a variety of such natural archives as tree rings, ice cores
and coral reefs. He also looks at anecdotal records found in old
letters, books and other documents. The strategy of using different
sources has many benefits. Most obviously, it helps Mann create
an unbroken record that goes back 1,000 years. We only use
records where the chronology is exact, he says.

This
approach also helps compensate for the limitations inherent in
each type of data. For instance, ice cores reflect temperatures
only in arctic regions and higher elevations when snow fell. The
layers of calcium carbonate found in coral reefs provide an indication
of climate conditions that is limited to tropical seas. Tree rings
are found everywhere except in tropical forests. The same limits
can be seen in the historical record. For instance, most historical
accounts of the Little Ice Age, a 400-year period
from approximately 1450 to 1850, were produced by people living
in Europe and eastern North America. Mounting evidence suggests
that these observations do not reflect conditions elsewhere.

Climate
change can occur regionally

Be
glad you werent living on Greenland in the late 17th
century. The island often was surrounded by ice and cut
off from nearby lands.

One
example of climatologist Michael Manns research in
patterns of climate change shows that only some regions
were affected during the period known as the Little
Ice Age. This resulted from a decrease in the suns
activity, which occurs periodically, causing a shift in
winds.

Mann,
assistant professor of environmental sciences at U.Va.,
and his colleagues at NASAs Goddard Institute for
Space Studies and at the University of Massachusetts, compared
results of a computer climate simulation with actual climate
data to estimate climate and atmospheric conditions during
the peak of the Little Ice Age. Their work shows that climactic
changes during the period were concentrated more regionally
than globally.

Changes
in the suns energy used to be one of the biggest factors
influencing climate change. Since the industrial revolution,
however, greenhouse gasses have become the biggest catalysts.

Between
the mid-1600s and the early 1700s, the Earths surface
temperatures in the Northern Hemisphere appear to have been
equal to or near the lowest levels during the 20th century,
and only about one degree Fahrenheit colder than today.
In contrast, European winter temperatures were reduced by
1.8 to 2.7 degrees Fahrenheit  and that difference
was enough to routinely freeze canals solid in Holland,
as well as lock up Greenland in ice.

In
an inverse way, this has implications for the global warming
debate, Mann said. It could suggest that when
global temperature warms even slightly, some regions would
experience potentially significant temperature increases,
which could affect people, crops and ecosystems.

By
using records from a variety of sources, we can assemble a much
more comprehensive, global picture, Mann says. By matching
these data with well-understood weather patterns like El Niño,
we can make reasonable surmises about conditions in areas for
which we have no data.

For
Mann, creativity involves moving deeper and deeper into a problem.
He found the level of accuracy attained by simply compiling data
was inadequate because it does not account for local anomalies
 conditions that might produce unusual readings in a specific
year or contaminate the data. Consequently, he chose to apply
the tools of multivariate statistical analysis, which enables
him to compare groups of data over any given year and compare
them with established patterns. He can then identify the most
reliable information in the available data.

We
turn to the instrumental record established during the past 100
years to identify a dozen or so influential patterns, he
says. In essence we are using the 20th century to train
the proxy data from previous centuries. To ensure that he
doesnt bias his analysis toward the 20th century, he assumes
that the frequency and amplitude of these patterns might be different
in the past.

Manns
creativity and persistence have paid off. Conclusions about yearly
temperature produced by his statistical methods have been confirmed
by climate models that incorporate such factors as variations
in solar energy, including the effects of these variations on
the jet stream.

This
validation, in turn, helps give the modelers the assurance they
need to apply their models to predicting changes in the future.

Main
story reprinted from the Winter 2002 issue of Explorations, produced
by the Office of the Vice Provost for Research and Public Service
and the University Development Office of Corporate and Foundation
Relations. Online at www.virginia.edu/researchandpublicservice/explorations

Clarification:
All of the articles reprinted from Explorations in the Feb. 8
issue of Inside UVA were written by Charles Feigenoff.